The illness has been compared to polio, which was nearly eradicated with the development of a vaccine in the 1950s. Both diseases cause “flaccid weakness” where people become unable to move their limbs and are often left paralyzed.

Polio was found to be caused by a gastrointestinal virus that lives in the intestines

AFM, however, often begins with symptoms of an upper respiratory infection that rapidly progresses to paralysis — as AFM attacks the spinal cord — and patients may have difficulty breathing.

The illness primarily affects children, and the cause is unknown.

The first outbreak in 2014 was linked with Enterovirus D-68 (EV-D68) — but since then, experts have not settled on one specific cause for the outbreaks.

It is suspected a specific virus or family of viruses such as EV-D68 may cause the outbreaks on the every-other-year pattern.

Inconsistent testing methods and voluntary reporting methods may leave the total number of cases underreported, experts say.